A battery is a compact, chemical source of voltage. A
variety of technologies are used (alkaline, Nickle-Cadmium,
etc.) in batteries, only some of which are rechargeable.
Note that rechargeable batteries tend to output voltages
significantly below the "nameplate" quoted voltage -- so for
instance, a rechargeable "9 V" battery can put out as little
as 7.2 V when fully charged.

There is, by the way, a very handy chart of battery
capacities here.
Meanwhile, Hawker Energy has a great site on "battery
basics" here.